Signal transition enhancement circuitry for use in color television signal processing apparatus is known from, for example, U.S. Pat. No. 4,553,042 and the article by Hartmut Harlos, "Picture Signal Improvement In Colour TV Receivers", published in the IEEE Transactions on Consumer Electronics, Vol. CE-31, No. 3, August 1985, both of which are incorporated herein by reference. The Harlos article describes an analog processing approach to transition enhancement and U.S. Pat. No. 4,553,042 describes primarily a digital approach to transition enhancement. In both systems, signal transitions having slopes greater than a predetermined value are detected. Responsive to the detected transitions, circuitry is employed to hold the initial amplitude value of the transition for substantially the duration of the transition and thereafter the final value of the signal transition is coupled to the signal output device. The transition is shortened to the time required to switch between the initial and final values of the transition, which time is typically much shorter than the original transition time. The Harlos system accomplishes this function by incorporating a series switch in the signal path followed by a storage capacitor having one terminal coupled to a point of fixed potential. The series switch is maintained closed until a transition is detected, at which time the switch is opened. The value of the signal at the beginning of the transition is stored on the capacitor and output to further processing circuitry. A predetermined time after the switch is opened it is returned to the closed condition to couple the incoming signal to the capacitor and the further proces ing circuitry.
In U.S. Pat. No. 4,553,042, input signal, represented by, for example, binary samples, is applied to a serial shift register. Responsive to the detection of a signal transition, samples are routed around ones of the shift register stages to effectively eliminate samples representing the transition per se. The eliminated samples are replaced with samples representing the end points of the transition so that the output of the shift register developes the signal transition in one sample period which is substantially faster than the original transition time.
A disadvantage of both of the aforedescribed systems is that they tend to delay the point in time that the transition occurs. A TV receiver that has such circuitry incorporated in the chrominance signal path must include compensating delay circuitry in its luminance signal path to account for the additional chrominance signal delay.
It is an object of the present invention to provide signal transition enhancement circuitry with reduced delay in the signal path.